1. Field of the Invention
The present invention relates to a bias potential generating circuit and particularly relates to a bias potential generating circuit which can be applied to an audio amplifier or the like.
2. Description of the Related Art
As a conventional circuit that generates a bias potential, a bias potential generating circuit 100 as shown in FIG. 12 is generally used. A bias potential generating circuit 100 shown in FIG. 12 is configured to include a switch SW1, a resistor R1, a resistor R2, and a capacitative element C1. One end of the switch SW1 is connected to a power supply VDD. One end of the resistor R1 is connected to the other end of the switch SW1. One end of the resistor R2 is connected to the other end of the resistor R1, and the other end of the resistor R2 is grounded. One end of the capacitative element C1 is connected to a node 1 which is a connecting point of the other end of the resistor R1 and the one end of the resistor R2, and the other end of the capacitative element C1 is grounded.
The bias potential generating circuit 100 having the aforementioned configuration allows generation of a desired bias potential, according to magnitude of respective resistance values of the resistors R1 and R2.
The operation of the above-described bias potential generating circuit 100 is shown in FIG. 13. At the time of startup, the bias potential generating circuit 100 turns on an enable signal EN, which is an on-off control signal of the switch SW1, to bring the switch SW1 into a conductive state. Accordingly, the capacitative element C1 is gradually charged with a charge from the power supply VDD. As a result, the bias potential of the node 1 gradually increases from the GND level to a predetermined bias potential. The rising time of the bias potential, namely, a time constant τ is determined based on the resistance values of the resistors R1, R2, and the capacitance of the capacitative element C1.
The operation of the bias potential generating circuit 100 at the time of shutdown is similar to that of startup. The bias potential generating circuit 100 turns off the enable signal EN to bring the switch SW1 into a non-conductive state. Accordingly, a charge accumulated in the capacitative element C1 is gradually discharged via the resistor R2. Due thereto, as shown in FIG. 14, the bias potential of the node 1 gradually decreases. In this case, the time constant τ is determined based on the resistance value of the resistor R2 and the capacitance of the capacitative element C1.
This kind of bias potential generating circuit is used for generation of a reference potential in, for example, an audio amplifier or the like. However, in a case in which this circuit is used in an audio amplifier, at the time of startup of the bias potential generating circuit, a POP noise may be generated.
Japanese Patent Application Laid-Open (JP-A) No. 2007-151098 discloses a control device that prevents pop noise at the time of startup by controlling a smoothing capacitor used for generating a reference potential by a PWM (Pulse Width Modulation) signal.
Pop noise is a noise originating in transition of a potential at the startup time of the bias potential generating circuit. Therefore, in order to obtain preferable transition of a potential, the resistance values of the resistors R1, R2, and the capacitance of the capacitative element C1 need to be increased, and the time constant τ also needs to be made longer.
However, due to the time constant τ being made longer, the costs of the resistors and the capacitative element would increase, and the size of the bias potential generating circuit would become larger.
For example, the control device disclosed in JP-A No. 2007-151098 is connected is a speaker. However, the speaker has a low impedance, and therefore, in order to increase the voltage to a reference potential, a transistor having a large size becomes necessary in a circuit to generate a reference potential of the control device. For this reason, a circuit area of the control device becomes larger, and the cost of the device increases. Further, a switching noise (EMI noise) may occur due to wiring of the speaker.
Further, JP-A No. 2005-217613 discloses pop noise preventing circuit as shown in FIG. 15. In this pop noise preventing circuit, in order to rise a reference potential to a predetermined bias potential Vref, two threshold potentials V1, V2 are fixed, and the reference potential is increased gradually from 0V, and after exceeding the threshold potential V1, the potential is made to rapidly become higher, and after the reference potential exceeds the threshold potential V2, the potential is made to gradually become higher.
However, in the circuit disclosed in JP-A No. 2005-217613, as described above, the reference potential cannot be changed smoothly before and after the time when the reference potential exceeds the threshold potentials V1, V2. For this reason, in the circuit disclosed in JP-A No. 2005-217613, pop noise may be still occur around the threshold potentials V1, V2.